This invention relates to an improved method for preparing tetrahydrofuran/alkylene oxide copolymers having low oligomeric cyclic ether content.
In the copolymerization of tetrahydrofuran (THF) and alkylene oxides to produce polyether glycols it is customary to use an acid-activated montmorillonite clay as a catalyst. This is shown, for example, in British Pat. No. 854,958, where the montmorillonite clay is referred to as a "bleaching earth". A clay of this type has ordinarily been activated by bringing it into contact with an aqueous solution containing only about 10%, by weight, or less of mineral acid. This replaces most of the clay's exchangeable sodium ions with hydrogen ions, but leaves most of the clay's exchangeable potassium, magnesium and calcium ions unaffected. While the process of copolymerizing THF and alkylene oxides using an activated clay catalyst of this type is generally satisfactory, it has been observed that the process also produces oligomeric cyclic ether byproducts, which can constitute as much as 10% to 15%, by weight, of the total product.
These oligomeric cyclic ethers are undesirable for several reasons. First, they are impurities and represent "dead" material to the purchaser of the copolymer product in the sense that 10% to 15%, by weight, of the material he buys is not useful for his purposes.
Secondly, when the copolymer products are used in the preparation of polyurethanes, their prime utility, the oligomeric cyclic ethers tend to degrade the polyurethanes' properties.
Last, the oligomeric cyclic ether byproducts are quite soluble in common organic liquids and so many cause processing difficulties in whatever uses are made of the copolymer products.